1. Field of the Invention
The present invention provides a method of immobilizing hazardous inorganic wastes for prolonged storage in a monolithic product created by the method. The invention, more particularly, relates to the use of apatitic crystalline structures in achieving these objectives and the resultant wasteform.
2. Description of the Prior Art
In recent years, there has been substantial concern about the deleterious effects on people, animals, and the environment of hazardous waste. This has resulted in major efforts to find ways of neutralizing or, otherwise, rendering harmless such waste.
The United States government statutes and regulations have provided legal standards for disposal of toxic inorganic wastes in such a manner as to preclude their entry into the ambient environment. The United States Department of Energy nuclear complex has directed substantial attention to such waste problems. Similarly, numerous states have laws dealing with this subject.
Among the particularly troublesome materials in respect of stable disposal, are the toxic heavy metals. Wastes of this type may occur in various physical forms including aqueous solutions, metal salts in sludge cakes, and metals intermixed with soils. Such hazardous metals must be concentrated and bound in a wasteform suitable for storage or permanent burial. Among the more prominent wasteforms are borosilicate glass and Portland cement-based grouts. In the case of glass, the waste is blended into ingredients that are melted at high temperature, molded into a convenient shape, and solidified. Grout formation is a low temperature process in which waste is mixed with Portland cement and ingredients, such as fly ash. A criterion by which a heavy metal immobilization method is considered suitable is that the wasteform pass the Toxic Compound Leach Procedure (TCLP) prescribed by the United States Environmental Protection Agency. See U.S. EPA, Solid Waste Leaching Manual (SW-924), Cincinnati, Ohio (1985).
A national repository for planned burial of high-level hazardous wastes stemming from weapons production and spent nuclear reactor fuel has been built underground in New Mexico, selected on account of the low risk of water ever seeping into the repository, contacting and solubilizing the wasteform and freeing the hazardous substances to migrate. Chemical durability that will resist leaching and dissolution for several centuries and millennia into the future is a key requirement. The tests and criteria were established by the United States D.O.E., Environmental Protection Agency, and the Nuclear Regulatory Agency.
Calcium phosphate materials known as apatites are naturally occurring minerals. The formula for stoichiometric apatite is Ca.sub.10 (PO.sub.4).sub.6 F.sub.2. One specie of the generic class of apatite compounds is hydroxyapatite which is defined as Ca.sub.10 (PO.sub.4).sub.6 (OH).sub.2.
It has been known to employ hydroxyapatite in connection with medical devices employable as synthetic bones or prostheses in the orthopedic medical field. See, generally, U.S. Pat. Nos. 4,612,053; 4,880,610; 5,047,031; 5,053,212; 5,129,905; and 5,178,845. Use of these materials in the medical environment does not involve reactants which are toxic. Also, the use in these prior art patents was directed toward establishing a composition similar to that of human bone.
Roy et al. "Crystal Growth and Phase Equilibria of Apatites," Crystal Chemistry, pp. 185-239, Academic Press (1978), discloses substitutions made in a hydroxyapatite structure.
It has been previously suggested to employ calcium phosphates in respect of various aspects of environmental restoration.
Gauglitz et al. discloses immobilizing uranium and thorium by adding pre-existing hydroxyapatites to brines containing various salts to form saleeite and meta-autunite from power plant waste. See "Immobilization of Actinides by Hydroxylapatite," MRS Symp. Proc. 257, pp. 567-573 (1991).
U.S. Patent 4,671,882 discloses a phosphoric acid/lime hazardous waste detoxification treatment process wherein heavy metals are precipitated from aqueous solutions by forming a sludge which is designated as non-hazardous. This process involves the acidification of H.sub.3 PO.sub.4 or acidic phosphate salts, adjusting the pH, adding a coagulant, adjusting the pH, and using a calcium source as dewatering. See, also, U.S. Pat. No. 4,707,273 which is directed toward manganese removal from aqueous industrial waste effluents. The manganese removal and removal of other heavy metals is said to be accomplished by acidification and phosphate addition followed by raising the pH and adding calcium. A calcium-manganese-phosphate complex is said to be precipitated in the presence of fluoride. It discloses precipitation of Ca.sub.3 (PO.sub.4).sub.2 CaF.sub.2 and CaF.sub.2.
U.S. Patent 4,442,028 discloses precipitation of hydroxyapatite as a powder by adding H.sub.3 PO.sub.4 and then Ca(OH).sub.2 to a heavy metal-containing liquid with the powder being introduced into Portland cement.
U.S. Pat. No. 4,737,356 discloses immobilization of lead and cadmium in solid residues from the combustion of refuse employing lime and phosphate. It discloses the addition of water-soluble phosphate, such as phosphoric acid to decrease solubilities of lead and cadmium. Ash from a flue gas scrubber, which is typically high in calcium content, is employed.
U.S. Pat. No. 4,442,028 discloses combining radioactive phosphoric acids with Ca(OH).sub.2 to form hydroxyapatite granules which are mixed with Portland cement to form a concrete. See, also, U.S. Pat. No. 4,537,710 and 4,847,008 which relate to storage of radioactive material. The latter patent discloses the use of lead-phosphate glass.
The use of borosilicate glass in vitrification of high-level nuclear waste is disclosed in Cunnane et al., "High-Level Nuclear Waste Borosilicate Glass: A Compendium of Characteristics," Vol. 294, Nat. Res. Sec. Symp. Proc. (1993). Storage of low-level radioactive waste in saltstone containing grout in vaults is disclosed in Seitz et al., "Near-Field Performance Assessment for the Saltstone Disposal Facility," Vol. 294, Mat. Res. Soc. Symp. Proc. (1993).
The solubility and speciation of plutonium (VI) in phosphate solutions is disclosed in "Bulk Solubility and Speciation of Plutonium (VI) in Phosphate-Containing Solutions," Vol. 294, Mat. Res. Soc. Symp. Proc. (1993).
U.S. Pat. No. 4,902,427 relates to a filter for removing heavy metals from drinking water and employs bone char to remove heavy metals by ion exchange wherein hydroxyapatite is employed as an ion exchange medium. See, also, Gauglitz et al., "Immobilization of Actinides By Hydroxylapatite," MRS Symp. Proc. 257, pp. 567-573 (1992), wherein it is indicated that the use of large enough quantities of hydroxyapatite as an ion exchanger reduced the content of heavy metals.
Treating waste water by precipitating hydroxyapatite as a method for removing phosphorous compounds from waste water is disclosed in U.S. Pat. No. 4,917,802. The method involves extracting calcium from calcium silicate hydrates in hydroxyapatite formation.
U.S. Pat. No. 3,032,497 discloses removal of strontium ions by means of ion exchange from an aqueous solution wherein hydroxyapatite is formed in the reaction.
It has been suggested that evaporator wastes containing sodium phosphates may be reacted with Portland cement and establish hydroxyapatite. Atabek et al., "Nuclear Waste Immobilization in Cement-Based Materials: Overview of French Studies," Sci. Basis for Nucl. Waste Management--XIII, MRS Vol. Proc. 176, pp. 3-14 (1990). The formation of hydroxyapatite by salt precipitation has been known. Such procedures produce a hydroxyapatite which is either in powder or gel form.
In spite of the foregoing disclosures, there is lacking in the prior art a teaching of an economic means for safe and effective disposal of large streams of toxic heavy metal waste and radioactive waste in a durable ceramic form which may readily be stored or buried for indefinite periods of time without meaningful risk of exposure to the ambient environment. There is also lacking a method for establishing monolithic storage structures within which the heavy metals have been trapped and wherein an apatite or apatite-like material has been employed as a binder in enclosing material. There is further lacking such a system wherein the potentially hazardous heavy metals and radioactive materials may be so entrapped in situ with resultant wasteform being monolithic in physical structure.